I don't see how it is unfair to blame the user for using a filter on another telescope of larger aperture that is wasn't designed for and having it fail. Right now the data that has been posted show misuse as the mode of failure but there is very little data at all so you can't come to any type of conclusion. If the filters are provided with a 4" reflector or 80mm refractor and the instructions state to use a cap that reduces the aperture to 60mm with the filter and user fails to do this, this is failure of the user. The original question was what is the reason why one shouldn't use these filters. We had failure when the filter was used on a telescope of larger aperture that the filter didn't come with. If the common mode of failure is misuse then we as a group need understand not to use them with a telescope they are not designed for and to also use them in correct manner on the telescope they were designed for. That is different then they are unsafe all the time. To make that determination we need data, not myths or second hand accounts.

(snip)....
when we run solar observing we always have a marshall, whose sole responsibility is to inspect each scope for safety, before and during the session. things that have been picked up vary from unfiltered finderscopes (which at best can result in burns to the forehead)....
Grendel

Slightly changing the subject, I once forgot to cover the finder on a C8 while solar viewing. This would have been a 6x30 finder. My forehead started to heat up. I moved around and saw bright glare coming from the finder, thankfully off to the side.

This is one of the few blessings of thinning hair. There wasn't any hair in the path to catch on fire!

I don't see how it is unfair to blame the user for using a filter on another telescope of larger aperture that is wasn't designed for and having it fail. Right now the data that has been posted show misuse as the mode of failure but there is very little data at all so you can't come to any type of conclusion. If the filters are provided with a 4" reflector or 80mm refractor and the instructions state to use a cap that reduces the aperture to 60mm with the filter and user fails to do this, this is failure of the user.

It's unfair because the fault of bad engineering is then shifted to be blamed as the user's error.

If the engineering standard were that the filters would cover the entire objective, with so much absorption of light per square millimeter, then an oversized filter would in fact work fine, so long as the user found a sure means to secure it. Using an undersized might then properly be ascribed as the user's error: Duh; it didn't cover the entire objective. Fairly obviously, something intended to cover something else really needs to cover it.

A standard sized solar filter at the ocular fails this test. Any user could, not just by carelessness but by reasonable, expected human error, put the wrong filter in the wrong scope. The failure is thus the engineer's, for not designing a system that an ordinary human can use without error, every time, or risk hurting if not blinding his eyes.

Hi All. Without a Hershel Wedge in conjunction with a solar filer it is not "safe" to view the sun with any size refractor telescope. Other than that you can use a solar rejection mask that installs over the objective, but I find a Hershel wedge and a Zeiss ND filter to be the best setup for high magnification viewing of Sun spots with any size refractor. Also the optical flats that Unitron used in there solar wedges are very good and seem to work as well as Optima BC or Bader wedges. As to taking readings of the heat generated at the focal plane of a telescope pointed at the sun. I can tell you the exact reading. Really Hot!!! (aveman

how it is unfair to blame the user for using a filter on another telescope of larger aperture that is wasn't designed for and having it fail

Another thought: We're asking a fair question, "Has anyone heard, firsthand, of anyone being blinded when a filter failed?" Analogously, I can say that I, personally, have never once seen a solar filter of the design in question labeled, "Use only with 60mm aperture," or, "For 90mm aperture," or "For 100mm aperture or smaller." If they were designed for differently sized scopes, they would all have different darknesses of glass, yet would all look identically opaque to the eye selecting them, and so would need to be labeled. If they were intended only for 60mm scopes, none are so labeled, nor is there warning to use an accompanying aperture mask, except possibly in a manual that should be read, but may not be. The situation invites user error, a high stakes blame game when one is engineering a telescope for viewing the Sun. The solar filters of this design that I, personally, have seen all say only, "Sun," and some of them are molded in red plastic instead of black.

Red instead of black plastic is an imperfect example of engineering for safety. With the red color (grey to the colorblind), there is less chance that the user would insert the black moon filter by mistake prior to looking at the Sun and, at the very least, risking getting hurt. The trouble is, that black moon filter would still fit. A better example would be designating dedicated solar telescopes (such as exist, some of which are mentioned in this thread), and making special focusers for them, with square solar filters that simply could not fit into ordinary telescopes.This is not perfect, because the user might still find a means for attaching the wrong type of focuser to a supposedly solar scope. Still, at least with the right focuser when it was installed, that wonderful person everyone knows who has endless curiosity about the natural world, but little mechanical ability, could never be brought to harm accidentally via an error with a solar filter.

Trouble is, putting the solar filter at or near the focus of a conventional telescope exposes it to a far greater risk of breakage due to thermal issues, so it is inherently safer simply to move it to the safest place. Where's that? Before the objective. What light does not enter the scope can not hurt anyone's eyes. Granted, as Grendel discusses, it must be mounted securely, and the finder must be blocked; but, this is still a safer situation, one engineered to evade the user's ability to err.

People take all kinds of risks for all kinds of good reasons. No one is saying that no one else should never look at the Sun through a telescope. In gymnastics, the padding at the base of the apparatus (balance beam, uneven parallel bars, and the like) must extend past where the user is likely to land to also cover the steel feet. Why bother? Isn't it user error when the gymnast falls in the wrong place? What about steel toed shoes in the workplace. Isn't it user error when the employee drops something, or steps where told not to? Why waste the money covering the drive belts in heavy machinery? Isn't it user error when clothing gets caught, pulling the operator into the belt? Why should the petroleum industry make certain that the pump for diesel fuel won't fit in your gasoline powered car? Isn't the user responsible for selecting the correct hose?

In Maine recently, a vendor's kerosene tank was accidentally filled with gasoline. This could have killed anyone who poured the fuel into a kerosene powered heater and lit it. Why the concern, with all the radio stations, newspapers, and Web sites spreading the news? Isn't it user error if someone smells gasoline instead of kerosene, and lights it anyway? What about people with no sense of smell? Aren't they responsible to always have someone else smell their fuel? In this instance, the error was caught early, so no one got hurt. It would not have occurred at all if, as with the diesel-versus-gasoline hoses for cars, the fill on the kerosene tank were engineered to be unique, so the vendor could not have made the mistake that put others at risk.

This forum is unusual in at once being a hobbyist's abode and, in the marketplace of ideas, a respected, authoritative source of information. If we say, "Use the filters," more people will; and, if we say, "Do not!" more people will not. If Jon's tests show that his remaining five or so filters do not fail under whatever conditions, that would be interesting to know, yet would not prove that no filter would ever fail out in the wild world of well intended, but possibly naive users. Given the flawed design of these old-style solar filters sitting near the focus, let alone the unknowns in the current quality of the materials in any particular filter as they all age well past thirty, forty, and even fifty years, we have a responsibility to tell the world that, as much as we love classic telescopes, we want astronomers to consign this badly engineered design to the history books and use modern filters when viewing the Sun. Equal caution and better solar filters can only result in fewer injuries, whether mild or severe.

I just started test (1, 60mm) at 10:25 PST. I was able to use an eyepiece barrel threaded into the star diagonal which allowed me to then thread the sun filter into the diagonal. That was a good suggestion. I will leave it pointed at the sun for a maximum of 2 hours. I have to run to a doctor's appointment at 11:00. I will check it before I leave and after I get back. This is the test that is most likely to succeed (i.e., not crack the filter of all the tests). Will report back later today on the result.

BTW - I have included a shot of the 60mm on my Atlas EQ-G so you can see what I am using for tracking for each of the tests.

Attached Thumbnails

Speaking of EP solar filters, aside from the cracking thing, how are they for actually blocking UV/IR? On occasion I've pondered adapting mine into a sort of objective solar filter for the front of a finderscope...

Interesting experiment... hopefully nothing else gets melted in the process! I have a cheap EP that was exposed to sun through a refractor, that had the solar filter taken off only for a few seconds, and it melted part of the interior lining of the lens holder. Lotsa energy in a focused beam!

Before experimenting on a larger diameter OTA, I'd be curious to learn the results of using a 60mm refractor with the SUN filter screwed directly into an eyepiece since many old unmodified 0.956” star diagonals did not have filter threads in them. At least the two Tasco diagonals I have (1960 & 1962 – both slightly different) don’t have threads in them.

Test (1, 60mm) ended at 1:40PM PST. A little over three hours. Skies are clear. The filter didn't crack.

I also took the advice of loosening the lock ring a little. I tested the temperature by touch of the filter and it was I would guess about 100-110 degrees F. I was able to hold it to my cheek (after a quick finger test) and it was hot but not so hot I had to pull it away.

Test (2, 60mm) ended at 3:50PM PST. A little over two hours. Skies are clear. The filter didn't crack.

I tested the temperature by touch of the filter and it was I would guess about 110-130 degrees F. I was able to hold it to my cheek (after a quick finger test) and it was hot but not so hot I had to pull it away. It was hotter than test 1, 60mm but not that much hotter. The eyepiece was much hotter though as expected.

The results of these first two tests are at once interesting and not surprising. If these filters cracked all the time, their uselessness would be clear. Instead, there are reports of people using them for years in their youth without problems, as well as verified reports of their cracking when exposed to apertures beyond 60mm. Given that Jon is testing just a few filters, not a statistically valid sample of thousands, in the end we will still have to decide whether to advise against using these antiques with live human eyes. I'll likely remain cautious (or a party pooper, depending upon one's point of view), and continue to say "No!" regardless of the results with these few filters. A well-secured, modern filter covering the entire aperture remains inherently safer, a desirable trait when the stakes are so high.

Looking forward to your continued results, Jon. Your work is a real public service, as well as good science and good fun.

Okay, so I used one of these SUN filters that came with my 60mm Towa back in the 70's and it never cracked when used with my 60mm Towa until later when I tried it on my 8" Cave, and it lasted less than a minute and I don't remember exactly what happened, but I had a 1.25" to .965" adapter and the K22mm .965 eyepiece with the solar filter was an ex-K22mm eyepiece that day. My excuse is that I was a teenager - glad I didn't burn my eyeball out! I haven't done any solar observing since...

On another sidenote - I did one time scare the ____ out of me when I was trying to find Jupiter or Venus during the day by using the Sun as a RA/Dec reference and then moving the 8" Cave tube to where the planet should be. My routine was to remove the finder and primary eyepieces from the scope, then adjust the setting circles so that when pointed at the Sun, the mount was aligned, then dial in the planet. The Sun was used as a reference for RA/Dec when pointed at the Sun and the tube's shadow was concentric. Well it worked great, but one time, I was moving the tube (with eyepieces removed) and somehow managed to pass that concentrated Sun beam across my face and one eye - the time was less than a second, but I still remember the heat, and I still have diminished vision in that eye, just a slight fault in color, blue things look green, green things look yellow, like that.

So, whether using these filters in a 60mm is dangerous or not is a myth, I sure would like to know. But surely, mis-using them is a clear danger!

This is a fine example of how scientific inquiry begins. A well designed, practical experiment determines whether a larger study of statistical significance is warranted. I like how you're raising the stakes bit by bit, so as not to destroy your few filters prior to collecting all the data you can. Despite the small sample, this will likely be the most thorough study ever conducted on this topic.

8" Cave... I was moving the tube (with eyepieces removed) and somehow managed to pass that concentrated Sun beam across my face and one eye - the time was less than a second, but I still remember the heat, and I still have diminished vision in that eye...

Amazing how many stories of near or (thankfully) relatively minor injuries are associated with accidents in solar viewing. Glad to hear you are pretty well okay, despite suspecting you'd rather still have perfect color vision in that eye.

My attitude toward safety stems from two sources. The first is extensive training in marksmanship as a child, with a superbly qualified riflery instructor. We never had any accidents on our range, but because we were disciplined, not lucky. We were taught always to treat every gun as though it were loaded. Optics during the day are like that. Every one is a potential loaded gun. Even birders must be careful, when tracking flying birds, not to pan their binoculars across the Sun. I still remember being taught that prior to being allowed to use binoculars as a child at summer camp.

Second is a quarter century of directing a children's summer camp. Wait long enough and fail to teach about safety, and any accident you can imagine will eventually occur. If not the exact accident, then a close variant that could have been prevented. Misfortune will befall not just the children, but the staff, too. Good fortune favors the well trained and thoughtful in their exercise of best practices.

This why I treat solar viewing so seriously, and believe that we modest and individually unknown star gazers -- who happen to inhabit the largest, most comprehensive, most respected astronomy forum -- have an obligation to advocate wise teaching.

I'm beginning to suspect that Jon's myth busting will show that these filters are more robust when used with 60mm objectives than I had feared. I nevertheless hold that, because we can not know the condition of any particular example of this type of solar filter, and the stakes are at least as high as avoiding Dan's class of injury, we would serve astronomy better by differentiating between what is interesting to know about these filters and what is wise to advocate in viewing the Sun.

Rock on, Jon! Let's see what it takes for you to bust your remaining filters!

Quite by accident I came across two article/letters in the S&T archive DVDs regarding the solar filters in use at the time. I was researching the book "Make Your Own Telescope" (Allyn J. Thompson) and looking up the articles associated with it. I came across an article by David Rosebrugh titled, "Safe and Sane Methods of Observing the Sun", in the October 1956 issue. In this article he basically says to not use the filter alone. He suggests reducing the aperture, use an uncoated mirror, use a prism (not oriented for total reflection), use a Herschel wedge, or use a partially transmitting metallic film in front of the objective along with the solar filter.

In the earlier letter, "Solar Filter Problem", in the May 1950 issue page 172, by Leo J. Scanlon, he mentions "being called upon to replace dense filter caps for use on solar oculars. This indicates that in many cases serious optical damage to the observer has been averted by a small margin." To summarize the letter he advocates not using the filter alone but in conjunction with other safety measures, Herschel wedge, etc.

These were the only two article/letters I could find about solar filter problems or usage in the 50's archive.

DISCLAIMER: I do not use or advocate the use of these older sun filters